I wouldn't feel safe sleeping in my car alone, or necessarily with another person, either. I might opt for a crappy Motel 6 over that, if you can't find a campground to pitch a tent in. Of course, I wouldn't get any sleep in a tent, either, although I have camped plenty of times. I did a 4 day tour where we camped, and it was very unpleasant having to ride after getting no sleep.
Can you investigate whether the ride sponsors could hook you up with a local person, who would let you stay at their house? I would trust a fellow cyclist over any of the other options.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport